The invention relates generally to the art of safety devices for pressure vessels and particularly to thermal-triggered actuation of relief devices for pressure vessels containing compressed gases.
Pressure vessels, such as those designed to contain compressed natural gases for use as vehicular fuel vessels, are equipped with pressure relief devices (PRD) which communicate with the interior of the vessel and, when actuated, act to vent the gases from the interior to decrease the pressure in the vessel or to route the gases safely away from the vehicle. Unfortunately, in the rare instances of fire or other localized heat event, pressure relief devices alone can be inadequate.
A variety of PRD""s are known and can be actuated thermally, by pressure, or by a combination of both. In the case of thick-walled metal vessels which have reasonably long endurance to elevated temperature, PRD""s which are actuated by pressure, are most commonly used. In the case of thinner, lighter pressure vessels used for fuel vessels temperature endurance is of concern. In such cases, pressure rarely builds to beyond containable levels before localized structural integrity is compromised. For this reason, fuel vessels are protected by thermally activated PRD""s or TPRD""s.
One such TPRD is disclosed in U.S. Pat. No. 6,006,774 to Lhymn et al, in which a creep-resistant, leakage free melt plug is formed from a low-melting matrix alloy dispersed with reinforcing agents. The low-melt alloy, when exposed to elevated temperatures such as from a fire, melts and allows gases to be vented through a connected venting tube.
Other thermally responsive safety valves, such as that taught in U.S. Pat. No. 4,827,962 to Rolfsen, use a gas and liquid filled vial located against a seat which normally blocks the outlet. The vial is frangible when heated to a predetermined temperature. When the vial breaks, the seat is permitted to be removed, causing the outlet to open, venting the interior before structural integrity is lost or pressures can build to explosive proportions.
In the case where vessels are long, such as in vehicular fuel vessels, problems arise because localized heat sources may be applied at locations remote or insulated from a TPRD, thereby resulting in localized rupture of the vessel without ever affecting or actuating the TPRD. This is especially true when portions of the vessel, often the end of the vessel carrying the TPRD and fill valves are physically separated or insulated from the reminder of the vessel by insulation or by the frame or chassis of the vehicle.
A recent newsworthy event in Canada graphically illustrates this problem. A stolen CNG-powered van was vandalized and it""s interior set afire. The steel fuel vessel, located in the chassis of the vehicle, was equipped with a single TPRD in the fill boss at the end of the vessel, which protruded outside of the vehicle. Because the temperature outside the vehicle remained below the actuation temperature of the TPRD it never did actuate. However, the extreme temperatures inside the van caused the vessel to fail and it""s contents to ignite explosively due to localized failure of the vessel. The explosion and fire resulted in loss of property and the potential of a loss of life.
In Canada, there is a requirement that vessels be fire tested, however the regulations for testing do not take into consideration that occasionally vessels are exposed to damaging localized heating which does not necessarily actuate the TPRD""s. Canadian Standards Association regulation B51-97 does require that all pressure vessels be tested with a uniform fire source every 1.65 m in length, providing direct flame impingement on the cylinder surface across its entire diameter so as to make sure that specified test temperatures are maintained.
Attempts have been made to resolve the problem related to fires at remote locations on the vessel relative to the TPRD. Most commonly, multiple TPRD""s and various pipes, conduits and associated venting lines have been located at a plurality of locations along the length and about the surface of the vessel in an attempt to cause actuation of the TPRD regardless the location of the fire. This has resulted in a significant increase in cost, both in manufacturing and assembly. Further, additional TPRD""s and their associated piping add significantly to the bulk of the vessel.
U.S. Pat. No. 5,848,604 to Eihusen attempts to resolve the problem by positioning a heat pipe substantially the length of the vessel from one polar end to the other and attached to a single TPRD at one end of the vessel. The pipe is designed to transmit heat as a result of a fire from a remote location on the vessel directly to the TPRD. The outer casing of the pipe is made from a thermally conductive metal such as copper, nickel or stainless steel and is lined with a wicking material, which operates on capillary action. The inside of the pipe is filled with a vaporizable fluid. When heat is applied to the pipe, the fluid, which has permeated the wicking material by capillary action, vaporizes and moves through the central core of the pipe, repeatedly condensing and vaporizing as it travels toward the PRD, until it transfers the heat to the PRD and actuates it.
To prevent serious injury or loss of property due to explosive ignition of compressed gases from failed vessels, as a result of fire, an economical and sensitive triggering mechanism must be provided and must ideally result in actuation of the PRD regardless of the location of the fire about the surface of the vessel and must do so in as short a time as possible.
The present invention provides a system for remote triggering of one or more thermal-pressure relief devices, the system being rapidly responsive to heat produced by a fire and the like.
More particularly, and as applied to a pressure vessel, a heat responsive fuse cord and ignitable by flame or heat, is positioned in the proximity of a protected pressure vessel and is thermally coupled to a thermal-pressure relief device (TPRD) which is in communication with the pressurized contents of the vessel. When ignited, as a result of localized flame or heat such as in a fire, the fuse cord burns to the TPRD, transferring the heat directly through a thermal coupler or through an intermediate initiator for actuating the TPRD. A kit comprising fuse cord and a thermal coupler provide an economical remote triggering system for TPRD""s having an otherwise limited environment of influence. A variety of forms of attachment can be provided to ensure the fuse cord remains in the proximity of the vessel.